High quality thin films for semiconductor, optical, magnetic and insulating utilities hays traditionally been formed by ICB methods. A typical prior art apparatus for use in such methods is disclosed in Japanese Patent Publication Number 54-9592 which is illustrated in FIG. 2. The prior art apparatus suffers from several significant deficiencies. First, when it is desired to produce a silicon or aluminum thin film, the vapors of these materials will react with tantalum and molybdenum which are typically used to form the ionization filament and electron extraction grid in the conventional apparatus. This reaction causes corrosion or erosion of both the grid and the filament which reduces their expected life and requires frequent replacement of same. Yet another problem which is encountered with the conventional apparatus in sputtering silicon or aluminum to form thin films is the good wettability of these materials with respect to the material normally used to form the melting crucible in the apparatus. As a result of the excellent wetting properties of the aluminum and silicon when in the molten state, they may creep up the inner walls of the crucible and exit in liquid form from the vapor ejection nozzle. This material is then free to creep along the top and down the sides of the crucible where it is ultimately vaporized in the space between the crucible and the heating element. As a result of this vaporization the impedance of the space between the crucible and heating element is lowered which results in this space becoming electrically conductive thereby preventing stable operation of the thin film forming apparatus. Yet other problems arise in the trapping of electrons emitted by the ionization filament in the electron drawing or attracting electrode. As a result, the ionization efficiency of the conventional apparatus is lower than would be desired.
The conventional prior art apparatus and its method of operation will now be illustrated with reference to drawing FIG. 2. In FIG. 2, Item 1 designates the vacuum vessel maintained at the predetermined, desired degree of vacuum; 2, represents the connection to the vacuum system for evacuating the inside of the vacuum vessel to the desired vacuum conditions; and 3, a vapor generating source accommodated in the vacuum vessel 1 at a lower position thereof. The vapor generating source 3 comprises a closed cylindrical crucible 4 having a nozzle portion 5 at an upper portion thereof, a heating filament 6 would in a solenoid shape and in the circumferential direction of the crucible 4 in order to heat the crucible 4, and a shielding plate 7 for shielding heat from the heating element 6. Material to be deposited is shown as Item 8 in crucible 4.
Item 9 designates the ionization means, which comprises an ionization filament which emits electrons and a grid 11 for accelerating the electrons emitted from the ionization filament 10, and a heat shielding plate 12 for shielding heat from the ionization filament 10. Item 13 designates the acceleration means which comprises an acceleration electrode 14 and an earth electrode 15 which causes the ionized clusters 33 to be accelerated by an electric field formed by the acceleration electrode 14 and the earth electrode 15. Item 16 designates the substrate which is disposed at the upper portion of the vacuum vessel 1 opposite from the nozzle 5 of the crucible 4. The substrate serves as the object on which the thin film 34 is formed.
A power source is designated as Item 20 which comprises a heating power source 21 for sending current to, and thus heating, heating filament 6; 22, a biasing power source for maintaining the crucible 4 at a positive potential with respect to the heating filament 6; 23 is an AC power source for heating the ionization filament 10. A DC power source, 24, is used to maintain the grid 11 at a positive potential with respect to ionization filament 10. An acceleration power source, 25, is connected between the earth electrode 15 and the acceleration electrode 14 in order to generate an electric field in the space between the earth electrode 15 and the acceleration electrode 14, and also to maintain the ionization means 9, as well as vapor generating source 3, at a positive potential with respect to the earth electrode 15.
The operation of the prior art apparatus will now be described. First, the desired vacuum is obtained and maintained through exhaust, 2, to achieve the desired vacuum. Usually, the vacuum is on the order of 10.sup.-6 Torr. Subsequently, the heating power source 21 provides power to heating filament 6. Electrons thus emitted from heating element 6 are accelerated by the electric field which is generated by biasing the power source 22, in the space between the heating filament and the crucible 4, and are collided with the crucible 4 thereby heating the crucible 4 and the substance for deposition 8 which is present in the crucible. Ultimately, the heating causes a portion of the substance 8 to evaporate and to be ejected upwards from the nozzle 5 forming a vapor flow 31.
When the vapor from the substance 8 passes through the space defined by the nozzle 5, undergoes accelerated cooling due to adiabatic expansion which causes the vapor to condense to generate groups of clustered atoms, called clusters. A portion of the clusters 32 are ionized by electrons emitted from the ionization filament 10 which has been heated by AC power source 23 and accelerated by grid 11, whereby a portion of the clusters are transformed into ionized clusters 33. The ionized clusters are accelerated by the electrical field formed by the acceleration means 13, and moved together with non-ionized neutral clusters 32 towards the substrate 16. The clusters collide with the surface of the substrate to form a thin film 34 thereon.
As can be seen from the foregoing description, the ionization filament 10 and the grid 11 are in the vapor flow and its possible during the process that a portion of this vapor wall deposit on the filament and grid which can cause difficulties, especially if the material is capable of reacting with the filament or grid. Often, the grid and filaments are made from tantalum or molybdenum which will react violently with materials such as silicon on aluminum which are often formed into thin films using this technique.
As is also apparent from the foregoing description, the electron drawing electrode is disposed between the ionization filament and the target clusters. Many of the electrons which are generated are captured by the electron-drawing electrode and thus do not provide for any ionization activity which reduces the overall efficiency of the ionization filament. Yet another problem with the conventional apparatus is the tendency of either the silicon or aluminum to wet the crucible and ultimately to become vaporized in the space between the heating electrodes 5 and the crucible 3 creating unstable operation which precludes the formation of a good quality film.
Accordingly, a need exists for a sputtering apparatus capable of a long life under stable conditions which operates at a high ionization efficiency.